1. Field of the Invention
The present invention is generally directed to a method and apparatus for electron pattern imaging and, more specifically, to the use of an image intensifier to amplify light in an electron backscatter diffraction (EBSD) pattern collection system in a scanning electron microscope (SEM) environment.
2. Description of Related Art
In the field of material microanalysis, EBSD is used to study the microcrystalline characteristics of materials. The method uses the electron beam of an SEM incident on a sample and an electron imaging system to collect the electron diffraction pattern.
With reference to FIG. 1, an example of a current arrangement for conducting EBSD is illustrated. The current arrangement includes a phosphor screen 1, lens (not shown), and a camera 3. The EBSD pattern is produced by illuminating electron beams 5 onto a sample 7 using the SEM 9. The EBSD pattern which falls on the phosphor screen 1 is converted to a corresponding light pattern which is focused by the lens system onto the camera 3. An EBSD map of the sample is produced by rastering the incident beam through an array of points on the sample 7 and collecting corresponding EBSD patterns at each point. EBSD sensitivity and collection speed is determined by at least one of the following: (a) incident electron beam accelerating voltage and current; (b) diffraction characteristics of the sample; (c) phosphor efficiency; (d) lens transmission; and (e) sensitivity and signal-to-noise ratio of the camera.
However, this current arrangement suffers from various deficiencies as discussed hereinafter. For example, SEMs typically operate at up to 30 keV accelerating voltages. However, it is more desirable to operate at much lower voltages and beam currents to minimize the interaction volume within the sample, thereby optimizing spatial resolution and minimizing damage and contamination of the specimen due to the high energy electron interaction. Under low voltage and low current conditions, most SEMs do not provide sufficient beam current for optimal illumination of the phosphor screen. Often accelerating voltage on the sample needs to be increased at the expense of spatial resolution and specimen degradation in order to obtain acceptable EBSD patterns.
Diffraction characteristics of the sample also create limitations in the current arrangement. The intensity of the EBSD pattern produced by the sample depends upon several factors: composition, degree of crystallinity, homogeneity, surface preparation, etc. These conditions diffuse and/or weaken the diffraction pattern incident on the phosphor screen.
The efficiency of the phosphor also plays an important role in the production of the EBSD pattern. Electrons interact with the phosphor material to produce light. The interaction volume is a function of the electron energy, phosphor composition, and thickness. The ideal phosphor needs to be very thin to minimize electron interaction volume, but sufficiently absorbing to convert all of the pattern electrons to light. An additional criteria is that the wavelength of light produced by the phosphor match the sensitive range of the camera.
In addition, lens clarity and F-number determine the amount of light transmitted from the phosphor to the camera. Geometric constraints limit the maximum size of lenses that can be used.
Finally, the sensitivity and signal-to-noise ratio of the camera also play a role in determining the quality of EBSD pattern obtained by the current system. Within the given constraints of sample characteristics and current technology (SEM current, phosphor, and lens efficiency) exposure time to achieve an acceptable EBSD pattern is fundamentally limited by sensitivity and S:N of the camera. This determines the minimum limit of pattern detection and maximum pattern collection speed.
Accordingly, a need exists for a method and apparatus for EBSD pattern collection that amplifies light passing from the phosphor screen to the camera without degrading the signal-to-noise ratio.